Self-contained mobile fueling station

ABSTRACT

A mobile self-contained self-powered station having a plurality of vessels delivers a pressurized fluid to a receiving tank (e.g., a fuel tank of a hydrogen-powered vehicle) without using mechanical compression, external electric power, or other external utilities. The station includes first and second vessels, a conduit in fluid communication with the receiving tank and each of the first and second vessels, means for transferring at least a portion of a quantity of the pressurized fluid from the first vessel to the receiving tank, means for measuring continuously a pressure differential between the increasing pressure in the receiving tank and the decreasing pressure in the first vessel, means for discontinuing the transfer from the first vessel when a predetermined limit value is reached, and means for transferring at least a portion of a quantity of the pressurized fluid from the second vessel to the receiving tank.

This application is a divisional application and claims the benefit ofpriority under 35 U.S.C. 120 of U.S. application Ser. No. 10/371,602,filed Feb. 21, 2003 now U.S. Pat. No. 6,786,245.

BACKGROUND OF THE INVENTION

The present invention relates to a method and system for delivering apressurized fluid, such as hydrogen or another compressed gas, to areceiving tank, such as a vehicle fuel tank, and in particular theinvention relates to a self-powered mobile fueling station fordelivering a fuel (e.g., hydrogen) at pressures of 5,000 psig or greaterto fuel tanks of vehicles, such as hydrogen-powered vehicles.

Although the invention is discussed herein with regard to delivery ofpressurized hydrogen gas to fuel tanks of hydrogen-powered vehicles,persons skilled in the art will recognize that the invention has otherapplications. For example, it may be used to deliver other pressurizedfluids which may or may not be used as fuels, and the pressurized fluidsmay be delivered to various types of receiving tanks other than vehiclefuel tanks.

With the increasing interest in clean and efficient fuels, automobilemanufacturers are designing and manufacturing hydrogen-powered vehiclesthat are powered by fuel cells or hydrogen internal combustion engines.Hydrogen is being tested in these vehicles and has the potential to bethe fuel of choice in the future.

These hydrogen-powered vehicles are in the development stage andmanufacturers are performing extensive tests to improve the vehicles andrelated technologies. Since there is not an established hydrogen fuelinginfrastructure in place, some manufacturers are installing fixedhydrogen fueling stations at test sites and elsewhere. Testing is takingplace throughout North America without sufficient capability to fuel thetest vehicles away from the fixed hydrogen fueling stations.

Hydrogen-powered vehicles are also being demonstrated and promoted atpublic events to increase consumer awareness and interest. These eventsare taking place at many locations where hydrogen fueling is needed butis not available. Currently, hydrogen is delivered to these events inthe form of liquid or as a cylinder product.

BX cylinders, individually or in packs, are typically used to providehydrogen to customers. However, these cylinders are very heavy anddifficult (expensive) to transport.

In view of the above, there is a need for mobile hydrogen fuelingstations to fuel test vehicles and demonstration vehicles at publicevents. Mobile hydrogen fueling stations also could be used formaintaining small fleets of hydrogen-powered vehicles, providing fuelfor emergency roadside assistance, and for fueling stationary fuel cellsor hydrogen-powered facilities at remote sites.

Powertech Labs and Dynatek, Inc. have offered for sale a mobile fuelingstation that is believed to have a supply pressure of 3,600 psig.

There exists a void in the availability of fuel for hydrogen-poweredvehicles. Government and industry demonstration projects are hampered bythe inability to fuel the prototype vehicles being tested anddemonstrated.

In view of the current needs of industry and government programs, amobile hydrogen fueling station is needed. Preferably, such a stationshould be a self-contained, self-powered, mobile fueling station capableof delivering high pressure gas (e.g., at pressures of 5,000 psig ormore) in an optimal manner (e.g., minimal fueling time and maximum usageof the fuel carried by the mobile fueling station so as to minimize theneed to refill the station).

As used herein, the term “self-contained” means that the power needed toactuate valves, deliver compressed gas at maximum pressure and atmaximum rates, provide communications between the fueling station and avehicle to be filled, and provide communications between the fuelingstation and a remote monitor is inherent in the fueling station. Theterm “self-powered” means that no external electric power or otherexternal utilities are needed to operate the fueling functions of themobile fueling station.

Although the prior art includes various types of mobile fuelingstations, none of these stations satisfy the current needs. For example,U.S. Pat. Nos. 5,983,962 and 3,257,031 each disclose a mobile fuelingstation. However, these patents do not teach how to deliverhigh-pressure hydrogen in an optimal manner.

Other patents and publications also disclose mobile delivery stationsfor storing and dispensing fuel, but these stations are not self-poweredand are not designed to deliver high-pressure hydrogen in an optimalmanner. See for example, U.S. Pat. Nos. 5,887,567 and 5,682,750. Seealso U.S. Patent Application No. 2002/0046773 and InternationalPublication WO 98/52677.

U.S. Pat. No. 5,596,501 discloses a system for dispensing fuel at remotelocations and a method of operating same. However, it does not teach amobile self-contained delivery station for delivering high-pressurehydrogen in an optimal manner.

The present invention teaches delivery of high-pressure hydrogen in acascading manner to optimize fueling time. Although the prior art doesdisclose cascading (e.g., U.S. Pat. Nos. 5,673,735 and 5,810,058), itdoes not disclose cascading delivery in an optimal manner (e.g., toprovide an optimal rate of fill) for use in a self-powered,self-contained mobile hydrogen fueling station.

U.S. Patent Application No. 2002/0014277 discloses an apparatus andmethod for filling a tank with hydrogen gas. However, it does notaddress the problems involved with filling tanks or storage vessels ofvarious sizes.

It is desired to have an improved method and system for delivering apressurized fluid, such as hydrogen gas, to a receiving tank, such as avehicle fuel tank.

It is further desired to have a method and system to allow for thefueling of hydrogen-powered vehicles in areas where there is no hydrogeninfrastructure (pipeline, plants, filling stations, etc.).

It is still further desired to have a self-contained mobile fuelingstation which can be deployed anywhere and provide fuel, such ashydrogen, to vehicle demonstration projects on an efficient, economicalbasis.

It is still further desired to have an automatic method and system tosafely store and dispense hydrogen gas at different pressures, making itpossible to fuel a vehicle rated for 5,000 psig or more without the useof a compressor.

It is still further desired to have a self-powered mobile hydrogenfueling station to support hydrogen demonstration projects and smallhydrogen-powered vehicle fleets without the use of external electricpower or other external utilities.

It is still further desired to have a self-powered mobile hydrogenfueling station which also may be used to provide emergency roadsideassistance to hydrogen-powered vehicles and/or to stationary fuel cellsor hydrogen-powered facilities at remote locations.

It is still further desired to have an improved method and system forcontrolling the rate of delivery of a pressurized fluid, such ashydrogen gas, to a receiving tank, such as a vehicle fuel tank.

It also is desired to have a method and system for delivering apressurized fluid, such as a hydrogen fuel, at a controlled rate ofdelivery to receiving tanks of various sizes, such as vehicle fueltanks, which afford better performance than the prior art, and whichalso overcome many of the difficulties and disadvantages of the priorart to provide better and more advantageous results.

BRIEF SUMMARY OF THE INVENTION

The present invention is a self-powered station and a method fordelivering a pressurized fluid from the self-powered station to areceiving tank without using mechanical compression, external electricpower, or other external utilities. The invention also includes anapparatus and method for controlling a rate of delivery of a pressurizedfluid from a storage vessel to a receiving tank through a conduit influid communication with the storage vessel and the receiving tank.

A first embodiment of the self-powered station has a plurality ofvessels, including a first vessel containing a first quantity of thepressurized fluid at a first pressure and a second vessel containing asecond quantity of the pressurized fluid at a second pressure. Thestation also includes: a conduit having a first end in fluidcommunication with a first receiving tank and a second end incontrollable fluid communication with each of the first vessel and thesecond vessel; means for transferring at least a portion of the firstquantity of the pressurized fluid from the first vessel through theconduit to the first receiving tank without using mechanicalcompression, external electric power, or other external utilities,thereby resulting in an increasing pressure in the first receiving tankand a decreasing pressure in the first vessel, the increasing pressurein the first receiving tank being less than the second pressure of thepressurized fluid in the second vessel; means for measuring continuouslya pressure differential between the increasing pressure in the firstreceiving tank and the decreasing pressure in the first vessel; meansfor discontinuing the transfer of the pressurized fluid from the firstvessel when a predetermined limit value is reached; and means fortransferring at least a portion of the second quantity of thepressurized fluid from the second vessel through the conduit to thefirst receiving tank without using mechanical compression, externalelectric power, or other external utilities.

There are several variations of the first embodiment of the self-poweredstation. In one variation, the pressurized fluid is a gas. In anothervariation, the pressurized fluid is hydrogen. In another variation, thelimit value of the pressure differential is zero. In yet anothervariation, the first receiving tank is a vehicle storage tank.

A second embodiment of the self-powered station is similar to the firstembodiment but includes means for moving the self-powered station fromthe first location near the first receiving tank to a second locationnear a second receiving tank.

A third embodiment of the self-powered station is similar to the firstembodiment but includes an insulation material disposed between thefirst or second vessel and a vessel adjacent the first or second vessel.

A fourth embodiment of the self-powered station is similar to the firstembodiment but includes a gas-permeable roof adapted to vent thepressurized fluid in a gaseous state.

The fifth embodiment of the self-powered station is similar to the firstembodiment but includes the following additional elements: means fordetermining when the plurality of vessels are empty or near empty; meansfor monitoring the self-powered station from a monitor in a remotelocation; and means for reporting to the monitor from the self-poweredstation a determination that the plurality of vessels are empty or nearempty.

A sixth embodiment is an automated mobile self-contained self-poweredstation having a plurality of vessels for delivering a pressurizedhydrogen gas at 5,000 psig or greater to a first hydrogen-poweredvehicle fuel storage tank without using mechanical compression, externalelectric power, or other external utilities. The station includes afirst vessel containing a first quantity of the pressurized hydrogen gasat a first pressure, and a second vessel containing a second quantity ofthe pressurized hydrogen gas at a second pressure. The station alsoincludes: a conduit having a first end in fluid communication with thefirst hydrogen-powered vehicle fuel storage tank and a second end incontrollable fluid communication with each of the first vessel and thesecond vessel; means for transferring at least a portion of the firstquantity of the pressurized hydrogen gas from the first vessel throughthe conduit to the first hydrogen-powered vehicle fuel storage tankwithout using mechanical compression, external electric power, or otherexternal utilities, thereby resulting in an increasing pressure in thefirst hydrogen-powered vehicle fuel storage tank and a decreasingpressure in the first vessel, the increasing pressure in the firsthydrogen-powered vehicle fuel storage tank being less than the secondpressure of the pressurized hydrogen gas in the second vessel; means formeasuring continuously a pressure differential between the increasingpressure in the first hydrogen-powered vehicle fuel storage tank and thedecreasing pressure in the first vessel; means for discontinuing thetransfer of the pressurized hydrogen gas from the first vessel when apredetermined limit value is reached; means for transferring at least aportion of the second quantity of the pressurized hydrogen gas from thesecond vessel through the conduit to the first hydrogen-powered vehiclefuel storage tank without using mechanical compression, externalelectric power, or other external utilities; means for moving the mobileself-contained self-powered station from a first location near the firsthydrogen-powered vehicle fuel storage tank to a second location near asecond hydrogen-powered vehicle fuel storage tank; means for determiningwhen the plurality of vessels are empty or near empty; means formonitoring the mobile self-contained self-powered station from a monitorin a remote location; and means for reporting to the monitor from themobile self-contained self-powered station a determination that theplurality of vessels are empty or near empty.

The present invention also includes an apparatus for controlling a rateof delivery of a pressurized fluid from a storage vessel to a receivingtank through a conduit in fluid communication with the storage vesseland the receiving tank. The apparatus includes: means for establishing apredetermined rate of pressure rise to be maintained during apredetermined time period for filling of the receiving tank with thepressurized fluid; and means for maintaining the predetermined rate ofpressure rise during filling of the receiving tank with the pressurizedfluid during the predetermined time period.

There are several variations of the apparatus. In one variation, themeans for establishing a predetermined rate of pressure rise includes acomputer/controller for generating an electrical signal convertible to alow pressure gas signal, and a regulator for amplifying the low pressuregas signal and controlling a fill pressure in the receiving tank.

In another variation, the means for maintaining the predetermined rateof pressure rise includes: a pressure control device in communicationwith the conduit or another conduit through which the pressurized fluidflows at an actual pressure before entering the receiving tank, thepressure control device adapted to increase or decrease the actualpressure of the pressurized fluid; means for calculating periodically arate of pressure rise over time; and means for commanding the pressurecontrol device to decrease the actual pressure when the rate of pressurerise is greater than the established predetermined rate of pressurerise, and to increase the actual pressure when the rate of pressure riseis less than the established predetermined rate of pressure rise.

In yet another variation of the apparatus, the rate of delivery iscontrolled as a function of either a percentage of a designated targetpressure already achieved or a percentage of a designated targetpressure yet to be achieved during a remaining portion of thepredetermined time period. In a variant of this variation, the functionis linear. In another variant, the function is geometric. In yet anothervariant, the receiving tank has an instantaneous thermodynamic statewhere the function varies over time with any changes in theinstantaneous thermodynamic state to provide an optimal rate of fill.

Another embodiment is an apparatus for controlling a rate of delivery ofa pressurized hydrogen gas at 5,000 psig or greater from at least onestorage vessel to a hydrogen-powered vehicle storage tank through aconduit in fluid communication with the at least one storage vessel andthe hydrogen-powered vehicle storage tank. This embodiment includes:means for establishing a predetermined rate of pressure rise to bemaintained during a predetermined time period for filling of thehydrogen-powered vehicle fuel storage tank with the pressurized hydrogengas, comprising a computer/controller for generating an electric signalconvertible to a low pressure gas signal, and a regulator for amplifyingthe low pressure gas signal and controlling a fill pressure in thereceiving tank; means for maintaining the predetermined rate of pressurerise during filling of the hydrogen-powered vehicle fuel storage tankwith the pressurized hydrogen gas during the predetermined time period,comprising a pressure control device in communication with the conduitor another conduit through which the pressurized hydrogen gas flows atan actual pressure before entering the hydrogen-powered vehicle fuelstorage tank, the pressure control device adapted to increase ordecrease the actual pressure of the pressurized hydrogen gas, means forcalculating periodically a rate of pressure rise over time, and meansfor commanding the pressure control device to decrease the actualpressure when the rate of pressure rise is greater than the establishedpredetermined rate of pressure rise, and to increase the actual pressurewhen the rate of pressure rise is less than the establishedpredetermined rate of pressure rise, wherein the rate of delivery iscontrolled as a function of either a percentage of a designated targetpressure already achieved or a percentage of a designated targetpressure yet to be achieved during a remaining portion of thepredetermined time period.

The present invention also includes a method for delivering apressurized fluid from a self-powered station to a first receiving tankwithout using mechanical compression, external electric power, or otherexternal utilities, the self-powered station having a plurality ofvessels, including at least a first vessel containing a first quantityof the pressurized fluid at a first pressure and a second vesselcontaining a second quantity of the pressurized fluid at a secondpressure. There are several embodiments and variations of the method.The first embodiment includes multiple steps. The first step is toprovide a conduit having a first end and a second end in controllablefluid communication with each of the first vessel and the second vessel.The second step is to place the first end of the conduit in fluidcommunication with the first receiving tank. The third step is totransfer at least a portion of the first quantity of the pressurizedfluid from the first vessel through the conduit to the first receivingtank without using mechanical compression, external electric power, orother external utilities, thereby resulting in an increasing pressure inthe first receiving tank and a decreasing pressure in the first vessel,the increasing pressure in the first receiving tank being less than thesecond pressure of the pressurized fluid in the second vessel. Thefourth step is to measure continuously a pressure differential betweenthe increasing pressure and the first receiving tank and the decreasingpressure in the first vessel. The fifth step is to designate a limitvalue of the pressure differential at which a transfer of thepressurized fluid from the first vessel to the first receiving tank isto be discontinued. The fifth step is to designate a limit value of thepressure differential at which a transfer of the pressurized fluid fromthe first vessel to the first receiving tank is to be discontinued. Thesixth step is to discontinue the transfer of the pressurized fluid fromthe first vessel when the limit value is reached. The seventh step is totransfer at least a portion of the second quantity of the pressurizedfluid from the second vessel through the conduit to the first receivingtank without using mechanical compression, external electric power, orother external utilities.

There are several variations of the first embodiment of the method. Inone variation, the first receiving tank is a vehicle fuel storage tank.In another variation, the pressurized fluid is a gas. In anothervariation, the pressurized fluid is hydrogen. In yet another variation,the limit value of the pressure differential is zero.

A second embodiment of the method is similar to the first embodiment ofthe method but includes an additional step. In the second embodiment,the self-powered station is mobile or portable and the additional stepis to move the self-powered station from a first location near the firstreceiving tank to a second location near a second receiving tank.

A third embodiment is an automated method for delivering a pressurizedhydrogen gas at 5,000 psig or greater from a mobile self-containedself-powered station to a first hydrogen-powered vehicle fuel storagetank without using mechanical compression, external electric power, orother external utilities, the self-powered station having a plurality ofvessels, including at least a first vessel containing a first quantityof the pressurized hydrogen gas at a first pressure and a second vesselcontaining a second quantity of the pressurized hydrogen gas at a secondpressure. The automated method includes multiple steps. The first stepis to provide a conduit having a first end and a second end incontrollable fluid communication with each of the first vessel and thesecond vessel. The second step is to place the first end of the conduitin fluid communication with the first hydrogen-powered vehicle fuelstorage tank. The third step is to transfer at least a portion of thefirst quantity of the pressurized hydrogen gas from the first vesselthrough the conduit to the first hydrogen-powered vehicle fuel storagetank without using mechanical compression, external electric power, orother external utilities, thereby resulting in an increasing pressure inthe first hydrogen-powered vehicle fuel storage tank and a decreasingpressure in the first vessel, the increasing pressure in the firsthydrogen-powered vehicle fuel storage tank being less than the secondpressure of the pressurized hydrogen gas in the second vessel. Thefourth step is to measure continuously a pressure differential betweenthe increasing pressure in the first hydrogen-powered vehicle fuelstorage tank and the decreasing pressure in the first vessel. The fifthstep is to designate a limit value of the pressure differential at whicha transfer of the first pressurized hydrogen gas from the first vesselto the first hydrogen-powered vehicle fuel storage tank is to bediscontinued. The sixth step is to discontinue the transfer of thepressurized hydrogen gas from the first vessel when the limit value isreached. The seventh step is to transfer at least a portion of thesecond quantity of the pressurized hydrogen gas from the second vesselthrough the conduit to the first hydrogen-powered vehicle fuel storagetank without using mechanical compression, external electric power, orother external utilities. The eighth step is to move the mobileself-contained self-powered station from a first location near the firsthydrogen-powered vehicle storage tank to a second location near a secondhydrogen-powered vehicle fuel storage tank.

A fourth embodiment is a method for delivering a pressurized fluid froma self-powered station to at least one receiving tank without usingmechanical compression, electric power, or other external utilities, theself-powered station having n+1 vessels, wherein n is an integer greaterthan zero, each vessel containing a quantity of the pressurized fluidhaving a pressure which decreases as the quantity decreases. This fourthembodiment of the method includes the following steps: (a) providing aconduit having a first end and a second end in controllable fluidcommunication with each of the vessels; (b) selecting the receiving tankto receive the pressurized fluid; (c) engaging the first end of theconduit in fluid communication with the selected receiving tank, theselected receiving tank having a pressure which increases as thequantity of pressurized fluid is delivered to the selected receivingtank; (d) selecting a vessel presently containing a quantity ofpressurized fluid at a pressure greater than a present pressure of thepressurized fluid in the selected receiving tank; (e) transferring atleast a portion of the quantity of the pressurized fluid from theselected vessel through the conduit to the selected receiving tankwithout using mechanical compression, electric power, or other externalutilities, thereby resulting in an increasing pressure in the selectedreceiving tank and a decreasing pressure in the selected vessel fromwhich the pressurized fluid is being transferred, the increasingpressure in the selected receiving tank being less than the pressure ofthe pressurized fluid in at least one other vessel; (f) measuringcontinuously a pressure differential between the increasing pressure inthe selected receiving tank and the decreasing pressure in the selectedvessel from which pressurized fluid is being transferred; (g)designating a limit value of the pressure differential at which atransfer of the pressurized fluid from the selected vessel is to bediscontinued; (h) discontinuing the transfer of the pressurized fluidfrom the selected vessel when the limit value is reached; (i) selectinganother vessel presently containing a quantity of the pressurized fluidat a pressure greater than the present pressure of the pressurized fluidin the selected receiving tank; (j) transferring at least a portion ofanother quantity of the pressurized fluid from the another selectedvessel through the conduit to the selected receiving tank without usingmechanical compression, electrical power, or other external utilities;(k) repeating steps (d) through (j) until the selected receiving tank isfilled with pressurized fluid at a desired filled pressure; anddisengaging the first end of the conduit from fluid communication withthe selected receiving tank.

A fifth embodiment of the method is similar to the fourth embodiment ofthe method, but includes the following additional steps: (m) selectinganother receiving tank to receive the pressurized fluid; (n) repeatingsteps (c) through (n) until the pressurized fluid can no longer bedelivered from the self-powered station to the last selected receivingtank without using mechanical compression, electric power, or otherexternal utilities.

A sixth embodiment of the method is similar to the fifth embodiment butincludes the additional steps of: (o) refilling at least two of the n+1vessels with the pressurized fluid, each refilled vessel containing aquantity of the pressurized fluid having a pressure which decreases asthe quantity decreases; and (p) repeating steps (b) through (p).

The present invention also includes a method for controlling a rate ofdelivery of a pressurized fluid from a storage vessel to a receivingtank through a conduit in fluid communication with the storage vesseland the receiving tank. The method includes two steps. The first step isto establish a predetermined rate of pressure rise to be maintainedduring a predetermined time period for filling of the receiving rankwith the pressurized fluid. The second step is to maintain thepredetermined rate of pressure rise during filling of the receiving tankwith the pressurized fluid during the predetermined time period.

There are several variations of the method for controlling the rate ofdelivery of the pressurized fluid. In one variant, the step ofestablishing a predetermined rate of pressure rise includes multiplesub-steps. The first sub-step is to generate an electric signalconvertible to a low pressure gas signal. The second sub-step is toamplify the low pressure gas signal. The third sub-step is to control afill pressure in the receiving tank.

In another variation, the step of maintaining the predetermined rate ofpressure rise includes multiple sub-steps. The first sub-step is toprovide a pressure control device in communication with the conduit oranother conduit through which the pressurized fluid flows at an actualpressure before entering the receiving tank, the pressure control deviceadapted to increase or decrease the actual pressure of the pressurizedfluid. The second sub-step is to calculate periodically a rate ofpressure rise over time. The third sub-step is to command the pressurecontrol device to decease the actual pressure when the rate of pressurerise is greater than the established predetermined rate of pressurerise, and to increase the actual pressure when the rate of pressure riseis less than the established predetermined rate of pressure rise.

In another variation of the method, the rate of delivery is controlledas a function of either a percentage of a designated target pressurealready achieved, or a percentage of a designated target pressure yet tobe achieved during a remaining portion of the predetermined time period.In a variant of this variation, the function is linear. In anothervariant, the function is geometric. In yet another variant, thereceiving tank has an instantaneous thermodynamic state and the functionvaries over time with any changes in the instantaneous thermodynamicstate to provide an optimal rate of fill.

Another embodiment is a method for controlling a rate of delivery of apressurized hydrogen gas at 5,000 psig or greater from at least onestorage vessel to a hydrogen-powered vehicle fuel storage tank through aconduit in fluid communication with the at least one storage vessel andthe hydrogen-powered vehicle fuel storage tank. This embodiment includesmultiple steps. The first step is to establish a predetermined rate ofpressure rise to be maintained during a predetermined time period forfilling of the hydrogen-powered vehicle fuel storage tank with thepressurized hydrogen gas. This first step includes several sub-steps.The first sub-step is to generate an electric signal convertible to alow pressure gas signal. The second sub-step is to amplify the lowpressure gas signal. The third sub-step is to control a fill pressure inthe hydrogen-powered vehicle fuel storage tank. The second step of themethod is to maintain the predetermined rate of pressure rise duringfilling of the hydrogen-powered vehicle fuel storage tank with thepressurized hydrogen gas during the predetermined time period. Thissecond step includes several sub-steps. The first sub-step is to providea pressure control device in communication with the conduit or anotherconduit-through which the pressurized hydrogen gas flows at an actualpressure before entering the hydrogen-powered vehicle fuel storage tank,the pressure control device adapted to increase or decrease the actualpressure of the pressurized hydrogen gas. The second sub-step is tocalculate periodically a rate of pressure rise over time. The thirdsub-step is to command the pressure control device to decrease theactual pressure when the rate of pressure rise is greater than theestablished predetermined rate of pressure rise, and to increase theactual pressure when the rate of pressure rise is less than thepredetermined rate of pressure rise. In this embodiment, the rate ofdelivery is controlled as a function of either a percentage of adesignated target pressure already achieved or a percentage of adesignated target pressure yet to be achieved during a remaining portionof the predetermined time period.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by way of example with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating an elevation view of oneembodiment of the self-contained mobile fueling station of the presentinvention;

FIG. 2 is a schematic diagram illustrating a plan view of one embodimentof the self-contained mobile fueling station of the present invention;

FIG. 3 is a schematic illustration of an end view of one embodiment ofthe self-contained mobile fueling station of the present invention;

FIG. 4 is a schematic diagram illustrating some of the piping andinstrumentation in communication with a programmable logic controller(PLC) in one embodiment of the present invention;

FIGS. 5A–5C illustrate a process flow diagram for one embodiment of thepresent invention;

FIG. 6 is a block-flow logic diagram illustrating the refill operationfor one embodiment of the self-contained mobile fueling station of thepresent invention;

FIG. 7 is a block-flow logic diagram illustrating the preliminary stepsfor initiating fill for one embodiment of the self-contained mobilefueling station of the present invention; and

FIGS. 8A–8C illustrate a block-flow logic diagram of a non-communicationfill for one embodiment of the self-contained mobile fueling station ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a self-contained self-powered mobile fuelingstation that satisfies a growing need in building a hydrogeninfrastructure. The fueling station leverages the benefits of cascadefilling to optimize the use of available pressures and volume to providean optimal rate of filling the fuel tank of a hydrogen-powered vehicle.Since the fueling station is self-powered, its delivery of fuel to thevehicle does not need any additional compression, and therefore does notrequire any hook-up to external electric power or other externalutilities.

Automatic filling of a vehicle tank by the fueling station is providedby use of PLC control of interlocked solenoid operated valves. Thevalves are actuated either by a portion of the hydrogen gas inventory orby a regulated gas from a dedicated high-pressure cylinder, which issafer than manual operation of the valves. Power required to operate thePLC is provided by a deep cycle battery, which is recharged byroof-mounted photovoltaic cells, regenerative (axle) generators, or arelatively small fuel cell powered by the system hydrogen inventory.

When the hydrogen inventory drops to a quantity where recharging thesystem is necessary, the unit communicates (wireless) with a monitoringfacility, letting someone know that it is time to retrieve the unit forrecharging. The use of composite materials in the storage vessels of thefueling station allows transport of the station to any region bylight-weight vehicles (<10,000 lbs.), and it therefore does not requireany special license.

The ability to provide fuel on a mobile platform permits extension ofhydrogen-powered vehicle demonstration projects with little costassociated with refueling. The self-contained self-powered mobilehydrogen fueling station can be deployed anywhere. The only requirementis a flat surface to park on. No electrical wiring is required. Noconstruction is required at the site, removing economic hurdles fordevelopment and additional demonstrations.

One embodiment of the invention is illustrated in FIGS. 1–9. Referringto FIGS. 1, 2 and 3, the self-contained mobile fueling station 20includes a mobile platform 14, an array of storage vessels 1, acomposite vessel support system 2, a composite vessel overpressureprotection system 3, actuating valves 4, a gas manifold 5, a flammablegas vent system 6, a dispenser panel including a PLC 7 mounted on adivision wall 26, a fueling hose 8 and nozzle 9, roof-mountedphotovoltaic cells 10, and a battery 11 for electric storage.

The fueling station 20 provides mobile fueling via a towable trailer(shown in phantom lines) having a gross vehicle curb weight less than10,000 lbs. This weight limitation permits towage by a single axlecommercial vehicle (e.g., ¾ ton pick-up truck). Other options include,but are not limited to, rail cargo, shipboard, or truck mounted systems.

Preferably, the storage vessels 1 are overwrapped carbon fiber compositevessels. Other lightweight large-capacity vessels also may be used.Lightweight vessels are used because standard steel or hoop wrappedsteel vessels have comparatively higher masses which are not conduciveto over-the-road transport by light vehicles (e.g., ¾ ton pick-uptruck). Overwrapped carbon fiber vessels have distinct advantages,because they are very stiff (high modulus), very tough, and are notsubject to many of the damage mechanisms (e.g., hydrogen embrittlement)as are steel vessels.

At least two storage vessels 1 are included in the array of storagevessels so that fueling can occur by cascading pressures, therebyallowing the highest possible differential pressures to develop, thusincreasing gas flow rates during vehicle tank filling, and minimizingthe time required for vehicle tank filling. Nine storage vessels areused in the embodiment illustrated in FIGS. 1–3.

The storage vessels 1 are supported by a support system 2 and alightweight frame (not shown) fabricated from an aluminum alloy,composite, or other high-strength lightweight material. The frame isconstructed to withstand a multiple of “G” forces, as required by DOTstandards. The support frame can fix the storage vessels by either strapmounting or supporting the storage vessels from the end “boss” fitting.The support frame also is designed to protect the valves (facing rear oftrailer) from shear, should the mobile fueling station 20 be involved ina collision with a stationary object or moving vehicle. It also isdesigned to let the fore end float relatively free in the longitudinaldirection, thereby mitigating any damage caused by expansion andcontraction due to temperature changes. Extension of the high-strengthlightweight frame beyond the valves acts to imbed the valve bodieswithin the skeleton of the frame, thereby protecting the valves fromimpact and shear loads.

FIG. 5 illustrates a process flow diagram for one embodiment of theinvention. A discussion regarding the instruments, valves, etc. isprovided below. FIG. 4 illustrates some of the instruments and valvesassociated with a storage vessel 1 and the lines by which signals aretransmitted between the PLC and those instruments and valves.

Referring to FIG. 5 (5A-5C), a first pressure transmitter 112 directlymeasures the pressure in the high-pressure manifold 5, and indirectlymeasures or reflects the pressure in any of the storage vessels (V1–V9)when the associated actuating valve 4 is open for a storage vessel.Pressure drop occurring at a first pressure control valve 110 is muchlarger than in the rest of the system, validating this assumption of theindirect measurement.

A second pressure transmitter 114 directly measures the pressure in themanifold 5 and hose 8 assembly downstream of the pressure control valve110, and indirectly measures or reflects the pressure in the vehicletank (not shown).

The pressure control valve 110 is used to regulate the flow rate ofhydrogen in the vehicle tank. Excessively high flow rates (and thus highpressure rates) in the vehicle tank generate heat that could damage thetank liner. Therefore, the pressure control valve maintains manageableheat input into the vehicle tank by maintaining acceptable rates ofpressure increase.

A temperature transmitter 116 is placed on one of the storage vessels 1,such as storage vessel V4, as shown in FIG. 5. The temperaturetransmitter may provide temperature data used in state equations tocalculate mass and density. A temperature-measuring device 115 providestemperature data for hydrogen in the vehicle tank.

Various valves are included in the system. A check valve 101 on eachstorage vessel (V1–V9) inhibits reverse flow of gas. A first hand valve131 is a manually operated valve for the on/off state. An electricallyactuated solenoid valve 102 associated with each storage vessel eitherpressurizes or depressurizes a gas-operated actuating valve 4 on eachstorage vessel. Miscellaneous check valves 104 and hand valves 136 shownin FIG. 5 are included for safety and control of flows in the system.

The mobile fueling station 20 is filled in the following manner. First,the PLC requests a signal from pressure transmitter 112 to determine ifthe residual pressure is above X psig, thereby verifying that there ispositive pressure and no ingress of air into the system. The PLC thenenergizes the solenoid valve 102 for storage vessel V1, allowinginstrument gas pressure to fill the instrument gas line 24 where servicegas pressure from the pressure relief device line 123 on storage vesselV1 is regulated in a second pressure control valve 125 down to 80 psig.The hydrogen at 80 psig then flows to the actuating valve 4 for storagevessel V1 causing the actuating valve to open. Upon opening, hydrogengas from the fill line 129 (connected to a hydrogen source via fillconnector 23) enters the high-pressure manifold 5 through a second handvalve 133 and flows into storage vessel V1 until pressure transmitter112 reads 7,000 psig. When pressure transmitter 112 reaches 7,000 psig,the pressure energizes the solenoid valve 102 for vessel V2, causingequalization between storage vessel V1 and storage vessel V2, and thenpressurization of both vessels back to 7,000 psig.

The process is then repeated. This time the solenoid valve 102 forstorage vessel V3 opens causing the actuating valve 4 for storage vesselV3 to open. Equalization then occurs between storage vessel V1, storagevessel V2, and storage vessel V3. This process is repeated until thelast storage vessel V9 is filled to 7,000 psig.

When the system reaches 7,000 psig with all actuating valves 4 for allstorage vessels (V1–V9) open, the PLC commands the solenoid valves 102for all vessels V1–V9 to de-energize, thereby closing the actuatingvalves 4 for all of the vessels V1–V9.

When the system is at full capacity, a third hand valve 135 is closedmanually. Hand valve 131 is closed, thereby completing isolation of allof the storage vessels (V1–V9) from the environment.

Several safety measures are incorporated in the system. For example,temperature activated pressure relief devices (12A, 12B, 12C) areprovided on each storage vessel (V1–V9). If the local temperatureincreases above 217° F., an internal melt plug liquefies, allowingmovement of the plug, causing a communication between the vesselcontents (hydrogen) of the storage vessel and the vent system.

Each storage vessel 1 also has a vent line on the bonnet of itsgas-actuated actuating valve 4. If the valve internals become damaged,escaping gas is piped to the vent system 6 instead of leaking into thetrailer vessel compartment.

Safety pressure relief device 139 is part of the overpressure protectionsystem 3, which protects the system from over pressurization. Thisdevice is set at about 6,350 psig and will lift upon system pressurereaching that value, thereby allowing hydrogen to be vented safely intothe dedicated vent system 6. The “dot” 150 on the discharge of eachsafety pressure relief device and check valve 101 indicates that thedevice outlet or valve is in communication with a vent header of thevent system.

Heat sensors 117–120 are provided in case of high temperatures (e.g.,due to fire) in the classified environment. Upon receiving a signal formthe heat sensors, the PLC will shut the system down.

Operation of the mobile fueling station 20 to fill a vehicle (not shown)is set forth below for the embodiment shown in FIG. 5. Hand valve 131 isopened to allow gas pressure to accumulate which can be used to actuatethe actuating valves 4 for each of the vessels (V1–V9) and the pressurecontrol valve 110. The PLC determines whether these permissives are inplace. If so, the fill process continues.

Pressure control valve 110 is opened to 7,000 psig. A signal to solenoidvalve 102 for storage vessel V1 energizes the solenoid valve for aboutone second, thereby allowing instrument hydrogen regulated to 80 psig toactuate the actuating valve 4 for storage vessel V1 for about onesecond. The hydrogen gas from storage vessel V1 pressurizes the manifold5 and hose 8 assembly for about one second. The PLC then sends a commandto de-energize the solenoid valve 102 for storage vessel V1, therebyclosing the actuating valve 4 for storage vessel V1.

The PLC repeats the process with the solenoid valve 102 and actuatingvalve 4 for vessel V2, pressurizing the manifold 5 and hose 8 assemblyat the higher pressure at the cascade step for storage vessel V2. Ateach step the pressure measured at pressure transmitter 112 is stored inthe PLC.

The process is repeated until storage vessel V9 has equalized with themanifold 5. The actuating valve 4 for storage vessel V9 is then closed.These steps validate the safety of the system, provide data to assessthe mass of hydrogen in each storage vessel, and pressurize the manifold5 and hose 8 assembly to a pressure high enough that equalization withthe vehicle (upon connection) will occur from a small volume into alarge volume (quickly).

The connection of the hose 8 with the vehicle is then made via hoseconnection 22. The manifold 5 and hose 8 depressurize, equalizing withthe vehicle tank to a steady-state value. The PLC monitors the pressuresat pressure transmitter 112 and pressure transmitter 114 making surethat no leaks exist.

The PLC then sends a signal to the solenoid valve 102 for storage vesselV1 to actuate the actuating valve 4 for storage vessel V1, allowing thecontrol of storage vessel V1 to depressurize into the vehicle tank. Whenthe differential pressure between the open vessel (measured at pressuretransmitter 112) and the vehicle (measured at pressure transmitter 114)is less than 200 psig, the PLC de-energizes the solenoid valve 102 forstorage vessel V1, closing the actuating valve 4 for storage vessel V1.

The PLC then commands the solenoid valve 102 for storage vessel V2 toopen, repeating the process, except at a higher pressure. Uponcompletion of this step, the equalization pressure will be at a higherlevel than in the step before. The process of opening and closingsuccessive valves is repeated until the fill is complete (density>1.5lbs per cubic foot) or the pressure in vessel V9 and the vehicle tankhave equalized at the highest pressure possible.

As shown in FIG. 4, the storage vessels are protected fromoverpressurization in two ways—by a safety pressure relief device 139 onthe vessel overpressure protection system 3, and by the array ofthermally activated pressure relief devices (12A, 12B, and 12C).

The safety pressure relief device 139 is in communication with thehigh-pressure manifold 5, which is in direct communication with theactuating valves 4 on each of the storage vessels (V1–V9). The pressurerelief device discharge is in communication with the flammable gas ventsystem 6, which exits the trailer through the roof 15. Alternatively,the roof may be made of a gas-permeable material, thereby reducing theneed to use the roof vent.

The thermally activated pressure relief devices (12A, 12B, 12C) areattached to each end of, and in the center of, each composite storagevessel 1. Each pressure relief device (PRD) is designed to discharge thecontents of its associated storage vessel when the temperature risesabove a predetermined limit (e.g., 217° F.), a situation that couldoccur if a fire existed. The low melting eutectic plug within the PRD isnot in the gas path, and therefore cannot re-solidify and block the gaspath. Each PRD is in direct communication with the high-pressurehydrogen contents of its associated storage vessel. The discharge ofeach PRD is vented to the flammable gas vent system 6, which exits theroof 15 of the trailer.

The piping of the flammable gas vent system 6 is designed such thatthere is limited backpressure in the system during a controlling event(e.g., safety PRD 139 opening, venting 7,000 psig hydrogen). It is alsodesigned to vent in a vertical direction and to prevent detonationwithin the non-purged vent pipe.

In an alternative embodiment, the center PRD 12B may be eliminated ifthe inter-vessel spaces within the bank of composite storage vessels 1is filled/stuffed with an insulation material (e.g., pearlite, glasswool). Use of insulation and/or other void filling insulation materials,including intumescent coatings, inhibit flame impingement on thecomposite storage vessels 1 should a leak occur. Also, removal of thecenter PRD 12B and the associated tubing reduces the probability of ahydrogen leak by reducing the number of fittings and length of tube/pipeunder constant hydrogen pressure. In this embodiment, lowering theprobability of leakage by reducing the number of PRD's, coupled withinhibition of flame impingement in the center of the vessel by use ofpearlite insulation, increases the margin of safety for the unit.

The inside of the fueling station 20 is considered a classified area,with the exception of a front storage space. An isolation wall 13prevents the migration of hydrogen gas into this area. All electricalcomponents in the classified area are non-sparking and comply with NFPAClass 1 Div. 2 group B requirements. The isolation wall 13 separates theclassified area 18 from the non-classified area 19, thereby allowing theuse of non-sparking components, as long as the components are at least15 feet away from the roof-mounted vent of the gas vent system 6. Theisolation wall is sealed, thus preventing ingress of hydrogen from thestorage vessels 1

A 12-volt electrical system powers the running lights of the trailer andenergizes the dispenser panel including PLC 7. The PLC is responsiblefor controlling the transfer of high-pressure hydrogen gas to the fuelstorage tank of a hydrogen-powered vehicle, and for controlling therecharging of the storage vessels 1 with hydrogen. The PLC performs thisfunction by activating the solenoid valves 102 which in-turn permit thepressurization of lines 28 feeding the actuating valves 4 on each of thecomposite storage vessels 1. The actuating valves are energized byhydrogen fed from the composite storage vessel PRD lines, but regulatedto 80 psig.

The PLC is powered by a 12-volt deep-cycle battery 11 is located in thenon-classified area 19 near the front of the trailer. The self-containedsystem provides its own mechanism for maintaining charge on thebattery—preferably a panel of photovoltaic cells 10 mounted to theexterior on the roof 15 of the trailer. The 15-volt output of this panelis wired to the battery, keeping it charged. As an option, a fuel cellmay be utilized for production of electricity. The fuel cell couldrecharge the battery, or could power the PLC on its own. (Other means,such as wind turbine generation or the power system of the“to-be-filled” hydrogen-powered vehicle could be used to maintain thebattery charge and energize the PLC.)

To use the mobile fueling station 20, hand valve 131 must be opened tosupply gas pressure to the solenoid valves 102. High-pressure hydrogenis reduced to 80–90 psig by pressure control valve 110 for actuation ofthe solenoid valves on each of the storage vessels (V1–V9). This actionis performed once the mobile fueling station has been disconnected fromthe tow vehicle. The actuating gas may be hydrogen from one of thestorage vessels, or it may be hydrogen or any inert gas (e.g., nitrogen)from an additional cylinder 141 dedicated to this task. In either case,the gas is delivered to the actuating valves 4 via the instrument lines144 shown in FIG. 5. The instrument lines vent at 145. As analternative, electrical solenoid valves could be used, provided theycarry a Class 1 Div. 1 or Class 1 Div. 2 group B NEC designation.

FIG. 5 illustrates both optional nitrogen actuation 16 and optional oralternative hydrogen actuation 14. The nitrogen actuation includes acylinder 142 of nitrogen gas which is connected to instrument line 24via connector 146.

The alternative hydrogen actuation 14 includes a cylinder 141, which isconnected to the primary lines for instrument hydrogen via line 148. Thehydrogen actuation system includes a surge bottle 147 and miscellaneouscheck valves 149 and hand valves 150 for safety and control of flows inthe system.

During filling of a fuel tank of a hydrogen-powered vehicle, thecommunication cable 17 hook-up to the hydrogen-powered vehicle isverified, acting as a permissive allowing the next step to commence.Communications may be wireless (e.g., infrared, radio frequency, etc.).A grounding connection is made between the self-contained mobile fuelingstation 20 and the hydrogen-powered vehicle using a provided cable (notshown). This prevents static discharge, which could serve as a potentialignition source.

Prior to initiating a fill by pressing a start button, entering apersonal identification number (PIN), or other process, the system readsdata coming in from the PLC. The PLC determines the pressure in each ofthe storage vessels 1 by assessing the signal coming in from acombustible gas monitor (not shown) within the classified area 18. If atthis time, or any time during the fill, the concentration of hydrogen issensed to be greater than 25% of the lower flammability limit (LFL) forhydrogen, the system shuts down. The program then asks the operator toverify that the hydrogen-powered vehicle to be filled (the “fillvehicle”) has been turned off.

To allow the system to operate, hand valve 131 permitting regulatedhigh-pressure hydrogen to energize the actuating valves 4 must beopened. Upon initiating the fill process, the PLC then sends a signal tode-energize each of the solenoid valves 102 controlling each of theactuating valves 4 on each of the storage vessels (V1–V9). A counter isreset, and a query asks the operator for his/her PIN. The operator isgiven three chances to input a correct PIN. After a third input of anerroneous PIN, the system will shut down, not allowing a fill to beinitiated. Upon input of a correct PIN, the program will ask theoperator to lift or rotate a lever, thereby grounding himself with thefueling station 20. (Grounding also may occur when the operator opensthe hatch-back door 21 to operate the unit.)

Also at this time the PLC scans the system for signals from the fillvehicle. If none are received, then the program commences with a“non-communication” fill. If signals from the fill vehicle are received,then a “communication” fill occurs.

In a “non-communication” fill, the pressure control valve 110 on thehigh-pressure manifold 5 is given a command to regulate gas to themaximum fill pressure. The PLC then commands each of the actuatingvalves 4 to open sequentially for a period of about one second. At eachof these steps, the storage vessel gas temperatures and the equilibriumpressure in the manifold 5 and the hose 8 are recorded and stored by thePLC. The stored values are further manipulated by use of the secondviral coefficients of hydrogen to produce the mass of hydrogen in eachstorage vessel (V1–V9). When the last actuating valve on the laststorage vessel V9 has cycled, the total hydrogen mass of the system iscalculated.

The fueling nozzle 9 is then attached to the fill vehicle, being surethat the integral “double block and bleed” connection is firmly in placeand locked in position. The PLC senses the status of this connection bymonitoring the pressure response of the manifold 5/hose 8 and thevehicle tank system. Verification of pressure stabilization providessafety interlocks for hose failure or leaks in the system. If the systempressure does not equilibrate, as monitored by pressure transmitter 112,then the system shuts down. When the system equilibrates, the values ofpressure and gas temperature (as measured by sensor 116) aremathematically manipulated to determine the mass in the vehicle tank.

The time required to fill the vehicle tank is calculated based on thedifference between the manifold 5/hose 8 equilibrium pressure and thefinal fill pressure required, and on the ambient temperature (measuredby temperature indicator 115). To mitigate overheating the vehicle tank,one of three fill rates is selected. If the ambient temperature is lessthan 15° C., the selected fill rate is 15 bar/min. If the ambienttemperature is between 15° and 30° C., then the selected fill rate is7.5 bar/min. If the ambient temperature is above 30° C., then theselected fill rate is 5 bar/min. The actuated pressure control valve iscontrolled by the PLC to provide the temperature dependent rate ofpressure increase as a linear function of time as determined earlier bydifferences in manifold and final fill pressure and the ambienttemperature.

The difference between the non-communication fill and a communicationfill is two-fold. First, communications signals from thehydrogen-powered vehicle fuel tank provide the PLC of the mobile fuelingstation 20 with instantaneous pressure and temperature signals insteadof relying on the hose/manifold pressure transmitter (PT-114) andambient temperature values. Second, the I/P control sub-routine providesfor a much larger pressure ramp rate in the communications fill whencompared to the non-communications fill. The ramp rate is maintained atmuch higher values until the temperature measured at thehydrogen-powered vehicle fuel tank reaches a high set point. Uponreaching this high temperature, the I/P controller commands the pressurecontrol valve 110 to temporarily pause at the instantaneous pressurelevel. The pause remains in effect until the instantaneous temperatureat the vehicle fuel tank has dropped to a value 5° C. below the setpoint, at which time the pressure ramp rate increases, returning to itsformer high filling rate.

The rate of pressure increase must be enabled by an increasing pressurein the manifold 5 to keep the pressure differential between the manifold(as measured by pressure transmitter 114) and the vehicle tank (asmeasured by pressure transmitter 112) significantly high to maintainflow rates that result in a quick fill. If the differential pressuredrops to a value less than 200 psig, then PLC commands are given toclose actuating valve (x) and to open actuating valve (x+1), therebysequencing the tanks and allowing a cascade arrangement to be afforded.The cascade fill arrangement has two major benefits:

-   -   1. cascade filling provides a greater number of fills at maximum        pressure; and    -   2. cascade filling minimizes of the time required for filling.

At the beginning of the initial fill, every storage vessel 1 is at itsmaximum design pressure. Upon connection to a fill vehicle tank, and thesubsequent opening of the first storage vessel, pressure equalizationbetween the fill vehicle tank and the first storage vessel occurs. Whenthe differential pressure across pressure control valve 110 is less than200 psig, the actuating valve 4 to the now “depleted” storage vesselcloses and the actuating valve on the next storage vessel is opened,allowing the process to repeat.

At each step, the pressure in the open storage vessel is decreased fromits starting value but is higher than the final pressure of the storagevessel before it in the sequence. Pressure increases from the firststorage vessel to the last storage vessel, as each storage vesselequilibrates at a higher pressure than the preceding storage vessel. Ifthe cascade filling scenario is not used, then the total pressure of thesystem will equalize at lower values during each fill until a pointwhere the highest system pressure will not be enough to fully fill avehicle tank.

In a cascade system, high pressure is conserved, as each subsequentstorage vessel equalizes at progressively higher pressures. Also, ateach consecutive step, compressed hydrogen flows at a rate significantlyfaster than experienced at the end of the previous step because of thehigher pressures encountered. This use of cascading pressures allowsfilling at a greater rate with less heating in the receiving tank whencompared to filling from one pressure.

While the filling is commencing, the system continues to sense for anybreaks in the hose 8, looking for depressurization (assessed by pressuretransmitter 112). Also, the system monitors the “virtual hydrogen flow,”a parameter being generated by subroutines using the above-describedparametric relationships based on hydrogen compressibility, systempressures, and gas temperature. Continuous computations are madecomparing the calculated mass loss from the trailer with calculated massgain in the vehicle. If at any time the virtual hydrogen flow exceeds avalue determined to be consistent with a full tank, a shutdown occurs.

Commands to adjust the I/P controller 27 on the programmable pressurecontrol valve 110 are given to maintain flow rates to support thepredetermined time to fill. If the instantaneous flow rate is above orbelow set values, the system shuts down. Pressure control valve 110receives its pressure input signals from the I/P controller, whichreceives mill-amp signals from the PLC. Its I/P pressure input signal isprovided by gas pressure, the gas being supplied from the same regulatedfeed to the actuating valves 4. This process is repeated until thedesired hydrogen density is attained in the receiving tank.

Hydrogen density is used as the control factor because it can bedetermined knowing only the pressure, temperature, and composition ofthe gas. Second order equations using viral coefficients help determinethe compressibility factor for hydrogen. The compressibility factor ofhydrogen provides a relationship between ideal and real conditions,allowing simple parametric equations to be used to calculate mass as afunction of pressure, volume, temperature, and composition. Use of viralcoefficients provides a convenient method of determining a precisemeasurement of the mass, and thus the density of hydrogen at differenttemperatures and pressures.

At the point where calculated hydrogen density exceeds a valueconsistent with a full tank, the fueling is finished and the PLC willnot allow additional hydrogen gas to flow. Filling is complete.

The interlocked solenoid-actuated, hydrogen energized on/off actuatingvalves 4 permit fueling to occur without requiring an operator tomanually open and close valves. Upon fueling, the PLC sends signals thatenergize valves associated with the particular step being performed. Allother valves are closed. The PLC program contains instructionsidentifying which solenoid valves 102 belong to which actuating valve(associated with which storage vessel). As the fueling occurs, aprogrammed sequence of valve openings and closings insures exactrepetition.

Use of manual valves would subject the operator to an enclosed spaceenvironment, which is inherently unsafe. Also, the use of manual valveswould place reliance on the operator to open/close the right valve atthe right time and in the correct sequence. Manual operation wouldintroduce operator error and require significant training. The PLCautomated interlocked valve control system of the present inventionreduces the amount of training required, eliminates or minimizes thepossibility of operator error, and allows for a faster fill.

In a preferred embodiment, the PLC also controls the commercial aspectof the fill. The operator (e.g., vehicle driver) is initially requiredto lift the hatch-back door 21 on the rear of the mobile fueling station20, thereby gaining access to the vehicle grounding conductor (notshown), the vehicle communication cable 17, the fueling hose 8 and thefueling control panel keypad (not shown). The raised hatch-back providesthe operator with some shelter from the elements (e.g., hot sun, rain,snow, etc.). The operator uses the grounding connector to create acircuit between the fueling station and the vehicle to be fueled,thereby mitigating the possibility of static discharge and possiblefire. The operator then enters a Personal Identification Number (PIN) atthe start of fueling. He/she also unreels and connects the communicationcable so that the PLC can assess the storage vessels 1, the initialtemperature and pressure of the fuel storage tank of the vehicle, andthe condition of the vehicle fuel system. This connection may bewireless in other embodiments.

Upon successful input of the PIN and connection of the communicationcable 17, the PLC verifies that the operator has successfully groundedthe vehicle, and that the nozzle 9 has been correctly placed/secured onthe vehicle. When these permissives are satisfied, the fueling begins.Simultaneously an account of the mass of hydrogen transferred isdisplayed on a screen (e.g., a LCD readout). The units of transfer canbe gallons, liters, or any other units. The charges may be ascribed tothe PIN's entity.

A preferred embodiment includes an automatic call-out system (not shown)for the recharging of the self-contained mobile fueling station 20 withhydrogen gas when it is near empty. A wireless transmitter (powered bythe electrical system of the fueling station) is activated when thehydrogen manifold pressure decreases to a predetermined value at thetier of highest pressure. Reception of this signal precipitates actionsleading to the retrieval of the self-contained mobile fueling station 20for recharging with hydrogen gas. It is also possible to monitor thefueling station in real time from a remote location to determine when itis near empty and should be recharged.

Recharging the storage vessels with hydrogen occurs at a centralfacility. The mobile fueling station 20 is parked near a compressoroutlet (not shown). An operator then attaches the communication cable17, and the grounding cable to the mating joints of the compressoroutlet. A flexible hose of the compressor station is connected to themating adapter 143 on the high-pressure manifold 5. An override for theinterlocked valve control system is provided, and can be accessed by useof a special code sequence on the keypad interface of the PLC. Theoverride opens each of the actuating valves 4 of each of the storagevessels (V1–V9) in sequential order starting with the lowest pressurestorage vessel V1 in order to minimize equalization pressure losses.This override significantly reduces the time and power required torecharge the mobile hydrogen fueling station. When the lowest pressurestorage vessel is at a specified target pressure, the valves are openedon each of the storage vessels at the next highest pressure. Anadditional permissive exists on the recharge system, such that if thepressure transmitter 112 reads less than 10 psig on the manifold, itwill not fill. This prevents the introduction of hydrogen into a storagevessel that may contain air, thus lessening the potential for energyrelease due to combustion. Also, similar to calculations made during thevehicle fill, the system can sense whether there is a leak or break inany of the lines or fittings. If when filling, pressure in the storagevessels does not increase at a specified rate over a given time period,the system will shut down.

The mobile fueling station 20 is in contact with the compressor used torefill the storage vessels 1. The same connection that is made with theto-be filled vehicle tank during a fill is made with the compressorstation. Upon reaching 7,000 psig, as measured at pressure transmitter112, the compressor station shuts down.

Upon completion of a normal refill, all of the solenoid valves 102 arede-energized, closing the actuating valves 4 of the storage vessels(V1–V9). Also, hand valve 131 on the supply gas to the solenoid valvesmust be closed prior to transporting the mobile hydrogen fueling station20.

FIG. 6 is a block-flow logic diagram illustrating a refill operation 200for one embodiment of the self-contained mobile fueling station 20.Terms corresponding to the abbreviations in FIG. 6 are: “PT” is pressuretransmitter; “HV” is hand valve; “P” is pressure; “N” is an integer from1 to 9; “AOV” is an actuating valve; “t” is time; and “ρ” is gasdensity.

In step 201 a communication link is established between the mobilefueling station 20 and a compressor station using the same communicationlink 17 that is used with hydrogen-powered vehicles. In step 202 a hoseconnection is made between the high-pressure manifold 5 of the trailerand the compressor discharge hose. In step 203 the compressor stationrequires a signal (pressure transmitter 112) from the PLC. Thisrequirement must be established to continue. If no signal is received,the refill cannot continue. If communication is established, the fillcontinues.

If no signal is received in step 203, the system is shut down in step204 due to a lack of communications between the compressor station andthe PLC of the mobile fueling station 20. If a signal is received, themanual valve (hand valve 131) must be opened in step 205 to provideactuating pressure to the actuating valves 4 of the mobile fuelingstation.

In step 206, a counter for the actuating valves 4 and associated storagevessels 1 is reset to 1 (the first vessel V1). Also, a minimum pressurevalue of 25 psig is input in step 206.

In step 207, the PLC commands actuating valve (N) to open for about onesecond to allow pressure transmitter 112 to determine if residualpressure exists in the storage vessel. If not, there is a highprobability of a leak. In step 208, the PLC determines whether there isa residual pressure of 25 psig for storage vessel V1, and whether thepressure cascades upwards, as it should, for each subsequent storagevessel. If not, then a leak is presumed and the system is shut down (asin step 204). Otherwise, the system is ready to proceed with a fill ofthe fuel tank of the vehicle.

In step 209, the PLC asks whether the last valve is open (i.e., thecounter is at 9). If not, it is necessary to loop back to step 207 viastep 210. In step 210, prior to loop back to step 207, the counter (N)is increased by one count, and the pressure value P is set to the lastpressure recorded in the high-pressure manifold 5. If a loop back wasrequired, steps 207, 208 and 209 are repeated until the PLC receives asignal that the last valve is open (i.e., the counter is at 9).

In step 211, the operator opens hand valve 133, providing communicationbetween the compressor station storage system and the storage system ofthe mobile fueling station 20. In step 212, the compressor starts,maintaining high pressure in the fill manifold.

In step 213, a second counter is set to 1. In step 214, the actuatingvalve associated with the present counter number is opened, allowing itto accept gas from the high-pressure manifold 5.

In step 215, the gas density (ρ) in the trailer storage system iscalculated using a parametric relationship based on pressure,temperature, and compressibility to determine the fill capacity of thestorage vessels 1. The manifold pressure is measured at pressuretransmitter 112, the storage vessel gas temperature is measured atsensor 116, and the compressibility factor associated with the measuredpressure and temperature is determined. If the density (ρ) is above aset point associated with the temperature compensated maximum fillpressure, then the shutdown sequence is initiated in step 219.Otherwise, the refilling continues in step 216.

In step 216, the PLC determines whether all of the storage vessels 1 areopen to the manifold. If the counter is not at the last vessel (V9),then the system proceeds to step 217 where the vessel counter isadvanced by one and the system loops back to step 214. (Steps 214, 215,and 216 are then repeated.) If the counter is at 9 in step 216, then thesystem proceeds to step 218.

In step 218, it is determined again whether the density in the storagevessels 1 is above a set point associated with the temperaturecompensated maximum field pressure. If it is, then a shutdown isinitiated in step 219 (where the compressor is shut down). Otherwise,the system allows time for further equalization.

In step 220, the PLC commands closure of all storage vessel gas operatedvalves, thereby breaking the communication with the high-pressuremanifold 5. Hand valve 133 is then closed in step 221, providing adouble block for the trailer system and the trailer refilling iscomplete in step 222.

FIG. 7 is a block-flow logic diagram illustrating the preliminary stepsfor initiating fill 300 of one embodiment of the self-contained mobilefueling station 20. The terms corresponding to the abbreviations in FIG.6 also apply to FIG. 7. Additional terms corresponding to additionalabbreviations in FIG. 7 are: “H2” is hydrogen; “V” is volume of thevehicle tank(s); and “LFL” is lower flammability limit.

Step 301 is the “start.” In step 302 the operator is asked to turn thevehicle off, thereby reducing a chance for sparks and combustion. Instep 303, a combustible gas monitor is asked to continuously monitor thestorage area atmosphere at the start (i.e., beginning with step 1). ThePLC asks whether a hydrogen concentration above the 25% lowerflammability limit (LFL) has been sensed or detected.

In step 304 (a “permissive”) the operator is asked to verify that thehand valve 131 is open and supplying gas to the actuating valves 4 andpressure control valve 110. In step 305, the PLC sends a signal todeenergize the solenoids associated with all of the gas-actuatedactuating valves 4 on each of the storage vessels (V1–V9).

In step 306 a PIN input associated counter is reset to zero. The counterrecords the number of times a personal identification number (PIN) iserroneously input. The operator is asked to input his/her PIN in step307. Then, in step 308, a decision is made to move forward or in thecase of an invalid PIN, to return to the PIN input screen. The counterrecording the number of invalid or erroneous “PIN” inputs is advanced byone in step 309. In step 310 a decision to allow another attempt at PINinput or to proceed to shut down is made based on whether the PIN wasinput three times erroneously.

In step 311 a timer is started, thereby inhibiting the operator fromfurther attempts to fuel. The time is set to approximately 10 minutes todiscourage an unwanted operator. The fill process is shut down in step312 and cannot be restarted for approximately 10 minutes.

In step 313 the operator is asked to establish a communication link 17with the vehicle. This may be performed by connecting a cable.Alternatively, a wireless communications link may be established usingradio frequency or infrared technologies. In step 314, a decision ismade. The fill can continue if the grounding lever is in the downposition. If the grounding lever was not down, instructions to lower thelever are given to the fueling operator in step 315. The operator isinstructed to lift the grounding lever in step 316, thereby dissipatingstatic electricity and reducing the potential for ignition.

In step 317 a decision on how to continue is made based on whether asignal has been received from the vehicle requiring the fill. The signalincludes the residual pressure (P) in the vehicle tank, the gastemperature (T), and the volume (V) of the tank.

In step 318, if a signal was received, then the program continues, butuses a subroutine associated with a “communications” fill. Otherwise, instep 319, if no signal was received, filling continues using a procedureassociated with a “non-communications” fill illustrated in FIG. 8 anddiscussed below.

FIGS. 8A–8C provide a block-flow logic diagram illustrating anon-communication fill 400 for one embodiment of the self-containedmobile fueling station 20. The terms corresponding to the abbreviationsin FIGS. 6 and 7 also apply to FIG. 8. Additional terms corresponding toadditional abbreviations in FIG. 8 are: “PCV” is pressure control valve;“Pf” is maximum fill pressure; “ID” is identification; “X” is an integerfrom 0 to 10 (one more than the number of storage vessels); “TE” istemperature measuring element; “Tamb” is ambient temperature; “tss” is“t” at steady state; and “S.R.” is sub routine.

In step 401 the non-communication fill start continues from thepreliminary steps for fill in FIG. 7 (i.e., from step 319 in FIG. 7). Instep 402 of FIG. 8, the pressure control valve 110 is opened to aposition associated with the maximum fill pressure. In step 403 thevessel identification counter is set to zero. The temperature of the gasin the vessel associated with the actuating valve associated with thefirst storage vessel V1 is read via sensor 116 and a minimum acceptablepressure for the actuating valve of any storage vessel is set at 100psig.

The vessel-sequencing loop is started in step 404 and advances thevalve(s)/vessel(s) by one. In step 405 a command is given to open agas-actuated actuating valve 4 on the associated storage vessel 1 in thesequence for one second. A decision is made in step 406. If the pressure(monitored by pressure transmitter 112) is less than 100 psig, then theunit shuts down is step 406A due to a system leak or empty storagevessel. This is to ensure that no air enters the manifold. If thepressure is greater than 100 psig, then the filling continues.

In step 407 a decision to continue is made based on whether the pressurein the storage vessel is greater than or equal to the pressure in thepreceding storage vessel. If it is (i.e., no leaks) then the systemcontinues with the fill. Otherwise, the fueling station 20 is shut downin step 407A.

In step 408, during the approximately one second, the equalized pressurein the vessel/manifold/hose is recorded and stored by the PLC. In step409, using the equilibrium pressure and the temperature of the gas asrecorded in TE-116, the values of the mass and density of the gas in thestorage vessel are determined and recorded. These values are calculatedknowing the storage vessel volume and the compressibility factor, whichis calculated using a temperature and pressure based parameter and thesecond order viral coefficient. In step 410 the mass and density for thestorage vessel are stored in a register that keeps track of the amountof gas in the storage vessel.

Another decision is made in step 411. If the system is not at the laststorage vessel (i.e., at the actuating valve 4 for vessel V9), then thesystem loops back and repeats the process for the next storage vessel(i.e., steps 404–411). On the other hand, if the system is at the laststorage vessel, then the system continues with the fill.

In step 412, the total mass of hydrogen in the system is calculated, andthat value is stored. Step 413 selects the pressurization ramp rate as afunction of ambient temperature (Tamb). If the ambient temperature isless than 15° C., then the system fills at a pressure ramp rate of 15bar/min. If the temperature is greater than 15° C., but less than 30°C., then the fill takes place at 7.5 bar/min. If the temperature ishigher than 30° C., then filling occurs at 5 bar/min. These ramp rateswere chosen to minimize heating of the vessel liner. At the higherambient temperatures, the vessel liners can be overheated, and the rateof heat increase must be controlled. This is done by reducing mass flowrate, mitigating heating by compression.

In step 414, the operator is instructed to make the “pressure-tight”hose connection. In step 415 the PLC determines whether upon making theconnection the manifold pressure (as measured by pressure transmitter114) drops to a value that is less than or equal to 95% of the pressurerecorded in the hose 8/manifold 5 at the end of the one-second cycles.If it has, then the system continues to step 416. Otherwise, it isassumed that the connection was not made, and the operator tries again.

In step 416 the pressure in the manifold/hose/vehicle tank is stored inthe PLC data bank as “PT-114 min.” In step 417 the PLC determines if thepressure is still dropping. If it is still dropping, the PLC asks again.Once the value at pressure transmitter 114 equilibrates (i.e.,satisfies >&=), then the filling continues.

In step 418 the timer is reset to zero. In step 419 the PLC questionswhether the pressure at pressure transmitter 114 is at least equal tothe minimum pressure stored in step 415 (“PT-144 min.”). If not, thenthe system proceeds to step 420. Otherwise, it proceeds to step 422.

In step 420 if the pressure is lower than “PT-114 min.”, the PLC askswhether the condition of continuously lower pressure has occurred for aperiod of 45 seconds. If not, then the loop starts over at step 415,establishing a new lower value of PT-114 min. If the condition hasoccurred for 45 seconds or more, the system continues to step 421. Instep 421, a determination of a slow leak has been established, and ashutdown occurs.

In step 422, the PLC determines whether the minimum pressure hasmaintained its value and determines whether the pressure has maintainedit value for 5 seconds. If so, the system is pressure tight, and thefill sequence advances to step 423, where the steady state pressure(P-114 SS) is recorded once the system has equalized. If not, it keepslooping back to step 419 until the 5-second period has been satisfied.

The equilibrium pressure in the manifold/hose/vehicle is established,and stored. From this value, an initial determination of mass can becomputed. In step 424 the connection is verified, and the fillingsequence continues.

Another decision is made in step 425. The PLC determines whether a“topping off” situation exists. If the pressure is greater than or equalto 4,500 psig, then the system goes to step 426. If the pressure is lessthan 4,500 psig, then the system goes to step 428.

At step 426 the fill sequence stops. No additional hydrogen will betransferred into the vehicle tank. The PLC display will show “FillComplete” in step 427.

In step 428, since the pressure is less than 4,500 psig, the filling cancontinue. The counter associated with the fueling station vessel valvesis reset to vessel 1 (V1).

In step 429 the PLC determines whether the grounding lever is still inthe lifted position. If so, filling continues (step 432). Otherwise, thesystem goes to step 430 (stop fill sequence). The grounding lever is away for the operator to terminate the fill, if required.

In step 430 the filling program is terminated due to the grounding leverbeing out of a normal fill position. The PLC will then display “LowerFill Lever” in step 431.

In step 432 the actuating valve 4 associated with the position in thecounter (1–9) is open, allowing hydrogen gas to pressurize the manifold5. The gas flows into the vehicle through pressure control valve 110 atthe pressure ramp rate prescribed, as based on the ambient temperature.

Another decision is made in step 433. The PLC determines if the pressureat pressure transmitter 114 is less than 75% of the value that itrecorded as the steady-state pressure in step 423. If it is, then a hosebreak is assumed and a shutdown is ordered, and the system is shut downin step 434 (hose break shut down). Otherwise, the system advances tostep 435.

In step 435 the timer used to maximize delta ΔP is reset (is ΔP<200 psigfor two seconds?). The density is calculated based on the pressure atpressure transmitter 114 and the ambient temperature. A decision is madein step 436. If the density (calculated by using the pressure atpressure transmitter 114, ambient temperature, and the hydrogencompressibility factor) is greater than 1.5 lbs per cubic foot, then thefill is terminated in step 437 (stop fill sequence). (In step 437, “FillComplete” is displayed on the PLC Display Panel.) On the other hand, ifthe density is 1.5 lbs per cubic foot, then the system proceeds to step438.

A decision is made in step 438. If the differential pressure between thevessel that is presently open to the manifold 5 and the vehicle(PT-112–PT-114) is less than 200 psig, then the flow is expected to beslowing down. If this is so, the system goes to step 439. Otherwise, itloops back to step 433, allowing further equalization to occur. Notethat in looping back the virtual flow totalizer is activated (see steps440–445).

Another decision is made in step 439 where it is asked whether thedifferential pressure (PT-112–PT-114) has been less than 200 psig for 2seconds. If it has, the system continues to step 446. Otherwise, itloops back to step 436, waiting for the 2 seconds to elapse.

In step 440, the value of hydrogen pressure in the manifold (PT-112) isstored. In step 441, using the second order viral equations, acalculation is made of the mass change in the storage system. In step442 the total mass change since initialization of the fill iscalculated.

A decision is made in step 443. If the total mass flow is greater than20 lbs, then the system goes to step 445 where the system is shut downdue to excessive flow (e.g., a leak in the vehicle tank). Otherwise, thesystem goes to step 444 where, using the value of mass transfer, and thepressures at PT-112 and PT-114, it calculates the step increase of theset point for the pressure control valve 110 required to maintain thepreviously input pressure ramp rate, thereby inhibiting overheating ofthe vehicle vessel.

In step 446 the value of the pressure in the open storage vessel 1 atthe end of its fill step is stored. In step 447 a command is given toclose the actuating valve 4 associated with the open storage vessel. Instep 448 the vessel/vessel valve counter is advanced by one, therebymoving onto the next step in the fill sequence.

A decision is made in step 449 where it is asked whether the counter isat 10. If it is, this means that for a 9-vessel system the end of thepressure cascade has been reached. If so, the system goes to step 426(stop fill sequence) and displays “Fill Complete” (step 427). Otherwise,the system loops back to step 429, allowing the permissives to beactivated to open the next higher-pressure storage vessel in thecascade.

Although illustrated and described herein with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the spirit of the invention.

1. An apparatus for controlling a rate of delivery of a pressurizedfluid from a storage vessel to a receiving tank through a conduit influid communication with the storage vessel and the receiving tank,comprising: means for establishing a predetermined rate of pressure riseto be maintained during a predetermined time period for filling of thereceiving tank with the pressurized fluid; and means for maintaining thepredetermined rate of pressure rise during filling of the receiving tankwith the pressurized fluid during the predetermined time period, saidmeans for maintaining the predetermined rate of pressure rise comprisinga pressure control device in communication with the conduit or anotherconduit through which the pressurized fluid flows at an actual pressurebefore entering the receiving tank, the pressure control device adaptedto increase or decrease the actual pressure of the pressurized fluid. 2.An apparatus as in claim 1, wherein the means for establishing apredetermined rate of pressure rise comprises: a computer/controller forgenerating an electrical signal convertible to a low pressure gassignal; and a regulator for amplifying the low pressure gas signal andcontrolling a fill pressure in the receiving tank.
 3. An apparatus as inclaim 1, wherein the means for maintaining the predetermined rate ofpressure rise further comprises: means for calculating periodically arate of pressure rise over time; and means for commanding the pressurecontrol device to decrease the actual pressure when the rate of pressurerise is greater than the established predetermined rate of pressurerise, and to increase the actual pressure when the rate of pressure riseis less than the established predetermined rate of pressure rise.
 4. Anapparatus as in claim 1, wherein the rate of delivery is controlled as afunction of either a percentage of a designated target pressure alreadyachieved or a percentage of a designated target pressure yet to beachieved during a remaining portion of the predetermined time period. 5.An apparatus as in claim 4, wherein the function is linear.
 6. Anapparatus as in claim 4, wherein the function is geometric.
 7. Anapparatus as in claim 4, wherein the receiving tank has an instantaneousthermodynamic state and wherein the function varies over time with anychanges in the instantaneous thermodynamic state to provide an optimalrate of fill.
 8. A method for controlling a rate of delivery of apressurized fluid from a storage vessel to a receiving tank through aconduit in fluid communication with the storage vessel and the receivingtank, comprising the steps of: establishing a predetermined rate ofpressure rise to be maintained during a predetermined time period forfilling of the receiving tank with the pressurized fluid; andmaintaining the predetermined rate of pressure rise during filling ofthe receiving tank with the pressurized fluid during the predeterminedtime period, wherein the step of maintaining the predetermined rate ofpressure rise comprises the sub-steps of: providing a pressure controldevice in communication with the conduit or another conduit throughwhich the pressurized fluid flows at an actual pressure before enteringthe receiving tank, the pressure control device adapted to increase ordecrease the actual pressure of the pressurized fluid; calculatingperiodically a rate of pressure rise over time; and commanding thepressure control device to decrease the actual pressure when the rate ofpressure rise is greater than the established predetermined rate ofpressure rise, and to increase the actual pressure when the rate ofpressure rise is less than the established predetermined rate ofpressure rise.
 9. A method as in claim 8, wherein the step ofestablishing a predetermined rate of pressure rise comprises thesub-steps of: generating an electric signal convertible to a lowpressure gas signal; amplifying the low pressure gas signal; andcontrolling a fill pressure in the receiving tank.
 10. A method as inclaim 8, wherein the rate of delivery is controlled as a function ofeither a percentage of a designated target pressure already achieved ora percentage of a designated target pressure yet to be achieved during aremaining portion of the predetermined time period.
 11. A method as inclaim 10, wherein the function is linear.
 12. A method as in claim 10,wherein the function is geometric.
 13. A method as in claim 10, whereinthe receiving tank has an instantaneous thermodynamic state and whereinthe function varies over time with any changes in the instantaneousthermodynamic state to provide an optimal rate of fill.